Subjects -> EARTH SCIENCES (Total: 771 journals)
    - EARTH SCIENCES (527 journals)
    - GEOLOGY (94 journals)
    - GEOPHYSICS (33 journals)
    - HYDROLOGY (29 journals)
    - OCEANOGRAPHY (88 journals)

GEOPHYSICS (33 journals)

Showing 1 - 31 of 31 Journals sorted alphabetically
Acta Geologica Polonica     Open Access  
Artificial Intelligence in Geosciences     Open Access   (Followers: 6)
Chinese Journal of Geophysics     Full-text available via subscription   (Followers: 1)
Contributions to Geophysics and Geodesy     Open Access   (Followers: 1)
Energy Geoscience     Open Access  
Eos, Transactions American Geophysical Union     Open Access   (Followers: 5)
Geodesy and Cartography     Open Access   (Followers: 2)
Geodesy and Cartography : The Journal of Committee on Geodesy of Polish Academy of Sciences     Open Access   (Followers: 2)
Geodesy and Geodynamics     Open Access  
Geofísica internacional     Open Access  
Geology, Geophysics and Environment     Open Access   (Followers: 1)
GEOMATICA     Hybrid Journal  
Geomechanics and Geophysics for Geo-Energy and Geo-Resources     Hybrid Journal  
Geophysical Research Letters     Full-text available via subscription   (Followers: 178)
GeoScience Engineering     Open Access  
Geothermal Energy     Open Access   (Followers: 5)
GIScience & Remote Sensing     Open Access   (Followers: 53)
Greenhouse Gases : Science and Technology     Hybrid Journal   (Followers: 4)
Interpretation     Hybrid Journal   (Followers: 1)
Journal of Earth Sciences and Geotechnical Engineering     Open Access   (Followers: 4)
Journal of Environmental & Engineering Geophysics     Hybrid Journal   (Followers: 3)
Journal of Remote Sensing & GIS     Full-text available via subscription   (Followers: 37)
Journal of the Earth and Space Physics     Open Access   (Followers: 9)
Near Surface Geophysics     Open Access   (Followers: 1)
New Zealand Journal of Geology and Geophysics     Hybrid Journal   (Followers: 6)
NRIAG Journal of Astronomy and Geophysics     Open Access   (Followers: 5)
Physics and Chemistry of the Earth, Parts A/B/C     Hybrid Journal   (Followers: 10)
Research in Geophysics     Open Access   (Followers: 5)
Results in Geophysical Sciences     Open Access  
Reviews of Geophysics     Full-text available via subscription   (Followers: 49)
Transportation Geotechnics     Full-text available via subscription   (Followers: 1)
Similar Journals
Journal Cover
Geomechanics and Geophysics for Geo-Energy and Geo-Resources
Number of Followers: 0  
 
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 2363-8419 - ISSN (Online) 2363-8427
Published by Springer-Verlag Homepage  [2469 journals]
  • Assessment of the Kiejo-Mbaka geothermal field by three-dimensional
           geophysical modelling

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      Abstract: High-resolution magnetotelluric and gravity data have been collected over the Kiejo-Mbaka geothermal field, located along the NW–SE trending Mabka fault, in the Karonga Rift basin (East Africa Rift System). Such resolution allowed to reconstruct the field structure with unprecedented detail. Resistivity modelling has been obtained by three-dimensional finite-differences inversion of MT data, while density modelling has been accomplished by surface-oriented inversion of gravity data. Geophysical modelling has identified two sedimentary sub-basins separated by the Mbaka fault ridge, exposing the basement; these previously unknown sedimentary fills have a maximum thickness of ca. 1.5 km. The estimation of the clay cation exchange capacity (CEC) from magnetotellurics identifies a layer of low-temperature smectite alteration in the south-western sub-basin sediments, interpreted as a clay cap. The resulting updated conceptual model of the Kiejo-Mbaka geothermal system is therefore a fault-controlled system with lateral leakage into the sediments, expectably implying a larger reservoir volume than previously estimated. Article highlights Geophysical survey of the Kiejo-Mbaka geothermal field (East Africa Rift System). Geophysical modelling depicted a secondary sedimentary reservoir. Outcomes deeply revised the resource assessment.
      PubDate: 2022-08-08
       
  • Investigation of the effects of two different cooling treatments on the
           physico-mechanical and microstructural properties of granite after high
           temperatures

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      Abstract: Granite deposits have a strategic importance due to the energy developments in Türkiye and its stability plays an important role in environmental safety. Therefore, it is of major importance for engineering practice to investigate the thermal damage mechanisms in the case of high temperature exposure effects. This study discusses the results of tests performed to investigate the physico-mechanical and microstructural properties of granite after exposure to high temperatures and two different cooling treatments. The samples, which were previously heated from 24 to 1000 °C temperatures and then cooled in an oven or water, were tested in terms of porosity, hardness, P–S wave velocity, uniaxial compressive strength, point load strength index (Is50) and microstructure. The Yaylak granite of Türkiye has been examined for the suggested research. The results show that the water-cooled granite samples exhibited higher decreases in P–S wave velocity, hardness, and uniaxial compressive strength compared to the oven-cooled samples. In addition, porosity values were increased in water-cooled granite samples. With scanning electron microscope and polarizing microscope analysis, the microcrack density and width in water-cooled granite samples increased more with high temperature, while they increased less in oven-cooled granites. The disintegration of the water-cooled granite samples increased considerably at 1000 °C and measurements could not be taken at this temperature. At the same temperature, water-cooling treatment caused microcracks and micropores, which led to more critical damage to the granite. These conclusions verify that different cooling methods have several effects on the physico-mechanical and microstructural properties of granite, and ensure a foundation for the prediction of rock mass behaviors in situations with high temperature exposure effects (e.g., geothermal exploration and nuclear waste disposal). Article highlights The physical and mechanical properties of granite are reduced after high temperatures with water and oven cooling methods. The effect of oven and water cooling increased the impacts on rock physical and mechanical parameters. The microstructural properties of rock are affected by the treatment temperature and cooling method.
      PubDate: 2022-08-08
       
  • Perforation cluster spacing optimization with hydraulic
           fracturing-reservoir simulation modeling in shale gas reservoir

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      Abstract: Understanding the hydraulic fracture propagation behavior of numerous clusters and its effects on final gas production is crucial for the successful development of shale gas reservoirs. This study introduces a methodology that combines hydraulic fracturing with the modeling of the production of a shale gas reservoir. A loose coupling approach is used to solve independent governing equations in the fracture and reservoir domains of two discretized separate domains. Complex fracture propagation paths are captured using the XFEM (extended finite element method) program. The simulation of shale gas production is also made more realistic by considering the Langmuir isotherm effect and non-Darcy flow. According to numerical models of hydraulic fracture propagation and shale gas production, the eventual shale gas output is sensitive to the actual hydraulic fracture path. Comparing the fracture geometry shows that the simultaneous fracturing design scheme has more complex fracture propagation than the sequential hydraulic fracturing design scheme. For the simultaneous fracturing design scheme, more intense stress interaction can result in stronger deflection. Whereas, in the sequential hydraulic fracturing design scheme, fracture deflection mainly occurs at the fracture tip area. Additionally, an increase in perforation may result in more fractures with deviating trajectories. Thus, it is more desirable to couple the curved hydraulic fracture stimulation and shale gas reservoir production modeling for the simultaneous fracturing design scheme. Because of the competition of closed hydraulic fracture, the final gas production is not positive with perforation number in both different fracturing schemes. The application submitted utilizing this approach shows that, in this particular scenario, gas generation using curved fracture geometry can provide greater performance than the typical straight fracture geometry assumption. Field operators may gain additional knowledge in choosing the best well completion scenario for a given set of geological parameters from the optimization fracture clusters along the horizontal well concerning production. Article Highlights An integrated hydraulic fracturing and shale gas production simulation approach was introduced. Optimization of fracturing design concerning shale gas production was proposed. Reducing cluster spacings needs a compromise with production and perforation numbers.
      PubDate: 2022-08-06
       
  • Numerical modelling of the performance of bolted rough joint subjected to
           shear load

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      Abstract: Abstract With the increasing complexity of engineering geological conditions, the problem of tensile-shear failure of bolts is becoming more serious. Accurate modelling of the performance of bolted rough joints subjected to tensile-shear load is essential for support design. In this paper, based on the theoretical criteria of bolt failure, the mechanical model of the Pile element in the FLAC3D program is modified, and the accuracy of the modified model is verified by numerical and experimental bolted rock joint shear tests. Then, the performance of bolted rough rock joints under shear load is studied. On the basis of the numerical modelling results, the interaction characteristics between bolt and surrounding rock, the distribution and evolution characteristics of bolt force and bolt shear resistance, and joint surface shear stress are revealed. Meanwhile, the influence of normal stress and joint roughness coefficient (JRC) are investigated. The research results are anticipated to give a useful guidance for the design optimization of bolting support for surrounding rock in underground engineering.
      PubDate: 2022-08-02
       
  • Study on the influence of stress differences on hydraulic fracturing based
           on true triaxial experiments and discrete element numerical simulations

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      Abstract: Abstract Laboratory experiments are an effective way to study the initiation and propagation of hydraulic fractures for the complex stress level and structure of underground reservoirs. However, it is difficult to observe hydraulic fractures directly in rock or mortar specimens. In this study, a kind of transparent rock-like material whose tension–compression ratio reaches 1/9.5 at normal temperature was obtained, which helped us to observe fractures directly. Using transparent specimens, triaxial hydraulic-fracturing experiments were carried out to explore the influence of stress differences on the initiation pressure and propagation of hydraulic fractures. The initiation pressure decreased with increasing stress difference, and the hydraulic cracks propagated along the maximum stress direction. The DEM (Discrete Element Method) simulation of hydraulic fracturing was used to verify the results derived from the laboratory experiments. This study provides new insights into the propagation of hydraulic fractures.
      PubDate: 2022-08-01
       
  • The role of creep deformation in pit lake slope stability

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      Abstract: Large-scale open-pit mining activities have profound impacts on the surrounding landscape and environment. At the cessation of open-pit mining, the rehabilitation of large void spaces can be achieved by pit-lake filling, where the water body provides a confining pressure on surrounding mine surfaces, reducing both the likelihood of slope failure and the need for ongoing slope maintenance. Although pit-lakes present a range of long-term benefits, the geotechnical performance of mines containing soft soils that are susceptible to creep under increasing loads due to pit-lake filling is seldom considered. From a geotechnical standpoint, creep induced failure is commonly associated with slow, downslope movements, prior to critical slope failure events. In this research, time-dependent slope stability analyses based on creep-sensitive materials are presented for an open-cut mine undergoing pit-lake filling. Numerical simulation provides a mechanism for the assessment of materials exhibiting soft soil creep constitutive behaviour under various loading conditions due to pit-lake filling. The response of mine surfaces is investigated for various filling regimes, highlighting location-dependent deformation rates, pore pressures and slope Factors of Safety for a large Australian open-pit brown coal mine. Results are presented for two separate creep-sensitive materials, identifying the ability to achieve final, stable landforms for a range of long-term pit-lake conditions. Article Highlights Time-dependent creep deformation behaviour is investigated for a large Victorian open-pit brown coal mine undergoing pit-lake rehabilitation. The soft soil creep model is implemented for a large open-pit rehabilitation model, to assess long-lasting creep movements of a specific mine slope. Mine void filling rates are simulated for a range of rehabilitation scenarios over a 5 to 40 year period, identifying the excess pore water pressure distributions in addition to vertical and horizontal deformations rates. The long-term behaviour of 8 cross-section profiles is presented, identifying the effect of pit-lake filling for silt and clay interseam materials.
      PubDate: 2022-07-29
       
  • Innovative measures for thermal performance enhancement of single
           well-based deep geothermal systems: existing solutions and some viable
           options

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      Abstract: Geothermal energy is taken as an important and future oriented energy source because of its renewable and clean features. The growth of geothermal exploitation and utilization, especially electricity generation, is still quite slow globally. The reason behind this situation can be attributed to high exploitation risk and cost, high water demand, and other environment related issues. The exploitation of deep geothermal energy could be more acceptable if the risk and cost can be effectively reduced. Therefore, a single well based geothermal system could be a possible choice, because the geological and hydrogeological uncertainty can be greatly reduced. However, a single well system is of limited area for heat transfer, thus usually resulting in low outlet temperature and unsatisfactory system efficiency. In the present work, existing methods for improving the heat extraction capability of a single-well geothermal system will be classified into different categories according to the theoretical model of borehole heat exchanger. The pros and cons of different methods are systematically analyzed, and some viable options that have not been investigated or paid attention to are also introduced with quantitative analysis. It is recommended to combine two or more methods together to achieve a better enhancement effect. Article highlights Various solutions for enhancing the thermal performance of single-well geothermal systems are grouped into four categories based on its governing equation, Targeted solutions were delivered for building an enhanced close- or open-loop system, Structured surface (STR), High-thermal-conductive Hollow Spiral Channel (HHSC), and hybrid LNG-BHE system can bring about remarkable performance improvement.
      PubDate: 2022-07-28
       
  • Influence of detonator delay scatter on rock fragmentation by bunch-holes
           blasting

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      Abstract: Abstract Bunch-holes blasting (several blastholes arranged in a bunch with short spacing and detonated simultaneously to form a large common blasting crater) is typically used in underground mines to increase blasting efficiency under a single free surface. Considering the low accuracy of pyrotechnic detonators, which are commonly used in Chinese mines, bunch-holes blasting with the same nominal times is actually short-delay blasting, and the actual delay intervals induced by the delay scatter of detonators can lead to unsuccessful bunch-hole blasting, which seriously affects rock fragmentation. In this study, theoretical analysis is firstly conducted to explore the mechanism of short-delay bunch-holes blasting, and the formation time of a new free surface is defined as the critical delay interval of bunch holes. Then, the formula of the new free surface forming time is deduced to determine the critical delay interval of the short-delay bunch-holes blasting. Considering the delay scatter of the detonator, the probability method is used to quantitatively analyse the probability of successful bunch-holes blasting with different delay nominal time detonators. Field experiments are performed to verify the results. This research demonstrates that as the detonator delay scatter increases, the probability of successful bunch-holes blasting generally decreases. In addition, the charge length also has an impact on the probability, which is positively correlated but not as dramatic as the detonator delay scatter. Hence, it is recommended to use high-precision detonators or low-segment detonators for bunch-holes blasting to ensure the quality of the cutting blasting.
      PubDate: 2022-07-27
       
  • Critical damage threshold of brittle rock failure based on Renormalization
           Group theory

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      Abstract: Abstract An in-depth understanding of damage threshold behavior during rock damage and failure is critical to predicting short- and long-term rock stability. The percolation theory was used to describe the rock damage threshold behavior and the Renormalization Group theory was used to accurately solve the theoretical solution of the critical threshold of brittle rock failure. By redefining the stress-transfer mechanism between rock blocks, the theoretical expression of the critical damage threshold of brittle rock failure was calculated based on the Renormalization Group theory. An experimental investigation of shale samples by in-situ micro-computed tomography was carried out to verify the theoretical expression. The values of damage calculated experimentally were consistent with the theoretical critical damage threshold of brittle rock failure. On this basis, the reasonable range limit of the critical damage threshold for rock mass failure was discussed. The laboratory test results on different lithologies and case studies of in-situ direct shear tests of rock and rockfalls in Mesnil-Val were analyzed to validate the feasibility of the threshold range and reveal the potential application of the research results.
      PubDate: 2022-07-26
       
  • Precursor of microseismic energy and stress evolution induced by rockburst
           in coal mining: a case study from Xiashijie, Shannxi, China

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      Abstract: Abstract Rockburst has become one of the most frequent dynamic disasters in coal mines when mining activities proceed towards greater depths. The goaf near the working face is the dominant factor, resulting in a high risk of rockburst and seriously threatening the safe production of coal mines. In this paper, to systematically analyze the stress and energy evolution of surrounding rock in the stope and reveal the rockburst precursory characteristics of the working face adjacent to the goaf, the microseismic (MS) monitoring technique and numerical simulation method were utilized based on the case of the Xiashijie Coal Mine in Shaanxi Province, China. The results indicated that the presence of the nearby goaf could lead to a dense high-energy event distribution in the overlying rocks owing to the fracturing of the hard main roof, which is closely related to periodic weighting and could be manifested as a periodic trend. Based on the source parameters of the energy index (EI) and cumulative apparent volume (CAV), lgCAV/lgEI could effectively evaluate the risk of rockburst and exhibits the advantages of convenience and thresholding. The risk of rockburst tended to increase with increasing lgCAV/lgEI. In particular, rockburst was prone to occur when lgCAV/lgEI exceeds 10. The distribution of the elastic strain energy derived via numerical simulation agrees well with that of the microseismic energy density measured via MS monitoring. The simulation results further indicated that when rockburst occurs, the average maximum principal stress in the main roof of the working face sharply dropped, while the average maximum principal strain significantly increased. Affected by the adjacent goaf, the advanced abutment stress of the roof and floor of the working face was distributed asymmetrically, with the characteristics of high stress concentration close to the goaf. Therefore, under dynamic disturbance due to notable energy release in the hard main roof, rockburst easily induced.
      PubDate: 2022-07-25
       
  • A micro–macro method for evaluating progressive and direct tensile
           fractures in brittle rocks

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      Abstract: Abstract A micro–macro method for evaluating the behaviors of direct tensile fractures during progressive loadings in brittle rocks is proposed in this study. The method consists of the suggested equation of the stress intensity factor of the mode-I crack that considers crack initiation, growth, and coalescence subjected to triaxial tensile loadings and the expression of axial strain relating to the extended length of the wing crack. The direct tensile correlation of stress and strain for depicting the complete initial elasticity, strain hardening, strain softening, and fracture stages is also studied. The reasonability of the presented method is proved by contrasting published results of experiment. Furthermore, the sensitivities of the density, inclination angle and size of the initial crack on the axial stress–strain curve, axial stress–crack length curve, tensile strength, crack initiation stress, and elastic modulus are determined. The tensile peak stress, tensile stress at crack initiation, and tensile elastic modulus descend with the increment of the inclination angle or size of the initial crack. The tensile peak stress initially descends and then remains constant, finally reaching a critical value with the increment of the density of the initial crack. The tensile elastic modulus descends with the increment of the density of the initial crack. The calculated results have a great significance for the safety evaluation of surrounding rocks in deep-buried underground engineering.
      PubDate: 2022-07-25
       
  • Cracking characteristic induced by supercritical carbon dioxide phase
           change in micro-cracks of coal

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      Abstract: The key to improving the efficiency of supercritical carbon dioxide (SC-CO2) jet drilling and completion is to clarify the mechanism of the SC-CO2 jet breaking coal. A large proportion of tiny coal powders appear during SC-CO2 jet breaking. However, there is no theory explaining this phenomenon. This study analyzed the cause of this phenomenon from the perspective of SC-CO2 phase change in the microcracks of coal. Scanning electron microscope experiments were used to compare and analyze changes in the coal crack scale and spatial position before and after the SC-CO2 phase change in the coal. The effect of the phase change of SC-CO2 at different temperatures and pressures on the particle size of coal powder was analyzed using a laser particle size analyzer. A low-temperature nitrogen adsorption experiment was conducted to clarify the changes in the pore size and volume of coal before and after the SC-CO2 phase change. The following conclusions are drawn: when the SC-CO2 jet breaks the coal, phase-change fracturing occurs in the fractures and pores. The phase-change action causes three phenomena: an increase in the width of the original crack structure of coal, mutual extension and penetration, and generation of new fractures. Fracture evolution process causes coal and cement to be dislodged, forming micropowders, the range of coal particle size is negatively correlated with temperature and pressure of SC-CO2. The SC-CO2 phase change inside coal pores increases pore size and accumulated pore volume. The effect of SC-CO2 phase-change fracturing is enhanced with increasing temperature and pressure. Article Highlights Supercritical carbon dioxide phase change acts on the coal and causes it to be destroyed at the microcrack scale. Supercritical carbon dioxide phase change causes coal fissures to widen, penetrate, extend, and create new cracks. The phase-change fracturing of a supercritical carbon dioxide jet breaking coal is the cause of coal powders.
      PubDate: 2022-07-23
       
  • Experimental investigation of mechanical properties, impact tendency, and
           brittleness characteristics of coal mass under different gas adsorption
           pressures

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      Abstract: Abstract Understanding the mechanical properties of coal after gas and coal interaction can better guide the mining technology of gas-bearing coal seams and drainage technology of coalbed methane. In this study, triaxial compression tests of coal mass under different gas adsorption pressures are carried out to evaluate the mechanical properties, impact tendency, and brittleness of coal with different gas adsorption pressures. Results show that due to the adsorption of gas, the mechanical parameters, such as elastic modulus and strength of coal are significantly decreased. In addition to the effect of fluid pressure, the decrease of elastic energy absorbed by coal, the increase of ductile deformation and the change of molecular structure of coal particles caused by gas adsorption also significantly affect the mechanical properties of coal. The influence of gas adsorption pressure on energy input becomes gradually evident when the coal enters the stage of plastic deformation and yield failure. Then the deformation and stress bearing capacity of coal are affected, and the energy accumulation and dissipation process of coal are further affected. The gas adsorption pressure generally develops the energy absorption density and elastic energy density of three characteristic points (initial damage point, volume expansion point and peak stress point), but reduces the dissipation energy density of these points. Gas adsorption reduces the storage of deformation energy of coal, strengthens the energy dissipation, and develops the plastic failure characteristics of coal. It is not prone to appear sudden brittle failure in coal mass. Although high pressure reduces the risk of rock burst, it increases the risk of gas outburst. The chemical effects of pore/fracture fluid caused by the combined action of adsorbed and free gas lead to the further decline in coal brittleness, thus changing the deformation and failure mode of coal.
      PubDate: 2022-07-20
       
  • Reservoir characterization of the middle Miocene Kareem sandstones,
           Southern Gulf of Suez Basin, Egypt

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      Abstract: Abstract In this study, we have assessed the petrographical and petrophysical characteristics of the progradational, syn-rift Middle Miocene Rahmi sandstone gas reservoir (Lower Kareem Formation) from the East Matr and Amal hydrocarbon fields, southern Gulf of Suez by integrating sidewall cores and wireline logs. We interpreted a reservoir gas gradient of around 0.09 psi/ft from the downhole pressure measurements. Based on well log-based petrophysical analyses, we interpreted that the Rahmi reservoir in the East Matr field has a 0.10–0.18 v/v total porosity, 0.08–0.14 v/v effective porosity, 0.08–0.17 v/v shale volume along with water saturation ranging between 0.09 and 0.32 v/v. The correlated reservoir in the Amal field is observed to have higher porosities (0.17–0.22 v/v total porosity and 0.15–0.19 v/v effective porosity), although it exhibits higher water saturation (0.38–0.54 v/v). The reservoir consists of very fine to coarse grained, poorly to moderately sorted, subangular to subrounded, poorly cemented and moderately compacted sublithic, subarkosic and arkosic arenites with moderate to good intergranular porosity. Abundant lithic fragments and poor textural maturity of the Rahimi sandstones imply a high energy shoreface depositional environment in close proximity to the hinterland. Porosity reduction is attributed to dolomite cementation, kaolinite, formation of pseudomatrix by mechanical compaction of argillaceous lithics, and quartz overgrowth. Long and concavo-convex intergranular contacts indicate that silica needed for quartz cementation was derived by moderate degree of chemical compaction of the quartz grains. Partial to near-complete dissolution of the labile grains (feldspar and lithics) and dolomite attributed to the reservoir quality improvement. Scattered dolomite cements prevented more severe mechanical and chemical compaction.
      PubDate: 2022-07-18
       
  • Developing the efficiency-modeling framework to explore the potential of
           CO2 storage capacity of S3 reservoir, Tahe oilfield, China

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      Abstract: Carbon capture and utilisation is a viable method of reducing greenhouse gas emissions. As a result, carbon dioxide (CO2) injection in oil formations is recognised as a promising solution for improving oil recovery factor whilst storing carbon in target sites. To achieve this goal, this study developed a novel efficiency workflow model to produce a reasonable geological model for exploring the potential CO2 storage capacity in the S3 reservoir of Tahe oilfield in China. The petrophysical properties of a well were initially predicted by artificial neural network. Then, object-based modelling technique was utilised to construct a lithofacies model. Afterwards, SGS and co-kriging techniques were employed to distribute petrophysical properties in a 3D geological model. Subsequently, 100 geological realisations were generated to assess the uncertainty of the pore volume. Thereafter, three ranked realisations (P10, P50 and P90) were utilised for uncertainty asessment of potential CO2 storage. Moreover, the CO2 storage capacity of brownfield was estimated to be in the range of 5.25–78.3 × 106 tons. Ultimately, this paper has clearly improved our understanding of potential for carbon storage and boost oil recovery in the S3 reservoir of Tahe oilfield. Article highlights Developing an integrated method considering OBM, SGS, co-kriging, and ANN for improving the S3 petrophysical distribution Assessment of the potential for CO2 storage in the S3 reservoir segment We provide the efficiency modeling framework for Tahe oilfield as the oil field has a similar characterization.
      PubDate: 2022-07-15
       
  • The ductile–brittle failure mode transition of hard brittle rock
           cutting—new insights from numerical simulation

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      Abstract: In rock mass cutting, the two main failure modes are ductile failure and brittle failure, and it strongly influence the rock breaking efficiency of the drilling bit. As such, investigation into the ductile–brittle failure transition (DBT) mechanism during rock cutting is crucial, especially for hard brittle rock. In the present study, a discrete-finite element model of heterogeneous granite was created and used for rock cutting experiments, so as to reproduce the cutting force and rock chip formation process. In addition, to interpret the DBT mechanism of granite cutting, theoretical models of mechanical specific energy (MSE) considering damaged zone, mean value of equivalent stiffness degradation (SDEG) and self-interlocking coefficient were developed. The study found that the cutting force bilinearly increases with the increase of cutting depth, and the cutter would crush the rock beneath the cutter tip. The average equivalent rock SDEG of the crushed zone reflects the damage value of rock chips. The critical transition depth (CTD) obtained using the theoretical models were almost equal. The self-interlocking of the crushed zone led to the secondary crushing of rock chips, which was the primary cause of ductile failure. Further, the self-interlocking was more obvious at large back rake angles and small depths, which consumed more energy. Reducing the self-interlocking of the crushed zone and increasing the ratio of brittle failure could effectively improve the rock breaking efficiency. The present study provides new insights from numerical simulations of DBT in rock cutting, and provides a basis for optimizing the cutter parameters of the drill bit. Article highlights A discrete-finite element model for heterogeneous granite according to Voronoi tessellation was established. Theoretical models of new MSE is proposed, which considers the damaged zone beneath the cutter tip. The characteristics of DBT from the perspective of cutting force, boundary effect, and rock chips morphology were reproduced numerically and experimentally. The causes of DBT of hard brittle granite were analyzed—self-interlocking and secondary crushing of chips.
      PubDate: 2022-07-15
       
  • Optimization of rock failure criteria under different fault mechanisms and
           borehole trajectories

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      Abstract: Abstract During the drilling process, wellbore instability leads to a complex wellbore morphology. Thus, serious drilling accidents, such as leakage and sticking, can occur. Therefore, it is important to choose different failure criteria for different geological engineering conditions to calculate the safe density window of the drilling fluid to prevent borehole wall instability. In this study, the applicability of three different rock failure criteria, Mohr–Coulomb, Mogi–Coulomb, and Drucker–Prager, was examined under five different fault mechanisms, and the variation in borehole collapse pressure with the trajectory of the borehole under different fault mechanisms was investigated. The results provided a scientific basis for the selection of failure criteria under different geological engineering conditions. The relevant theory provides a reference for well trajectory design, well location selection, and drilling directions.
      PubDate: 2022-07-12
       
  • Effect of pre-existing cracks on thermal cracking of granitic rocks under
           confinement

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      Abstract: Abstract Pre-existing cracks affect the thermal, hydraulic, and mechanical behavior of rocks. However, little effort has been devoted to investigating thermal cracking interactions with pre-existing cracks. In this paper, we propose a novel thermo-mechanical grain-based finite discrete element method approach to investigate the influence of pre-existing cracks on thermal cracking processes under confining pressure in granitic rocks. An intact synthetic granite sample was mechanically and thermally calibrated against experimental data for Stanstead granite, and petrographic thin sections of thermally treated granite up to 380 °C were used to qualitatively calibrate the thermal cracking pattern in terms of cracking evolution with respect to mineral grains. Then, we introduced varying levels of pre-existing cracks density to the calibrated synthetic sample to study the emergent thermo-mechanical behavior under unconfined and confined conditions. Additionally, we studied the effect of varying friction coefficients of pre-existing cracks on the thermal cracking process. Results show that thermal cracks preferably initiated from the tips of pre-existing ones due to stress concentration and the number of initiated thermal cracks decreased with increasing pre-existing cracks density. A decreasing trend of thermal cracking with confinement was also observed. Interestingly, increasing pre-existing crack friction increased the number of thermal cracks under different confinement conditions due to increased stress concentration along cracks interfaces. The results of this novel numerical process contribute to a better understanding of failure mechanisms in granitic rocks, particularly in geothermal reservoirs and underground nuclear waste repositories under various thermal and mechanical loading conditions.
      PubDate: 2022-07-11
       
  • Experimental studies on physical and mechanical behaviors of heated rocks
           with pre-fabricated hole exposed to different cooling rates

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      Abstract: Abstract In enhanced geothermal systems, thermal shock occurs around the borehole when cold water is injected into the hot rock of a geothermal reservoir. Experiments were performed on granite and sandstone specimens to simulate and investigate the effects of temperature and cooling rate using five temperature gradients and two cooling methods. Scanning electron microscopy was applied to examine the microstructural changes after thermal treatment, and a series of uniaxial compress tests were conducted on rock specimens containing a simulated borehole. The digital image correlation (DIC) method was applied to analyze the crack evolution during loading. The test results show that the thermal damage induced by the propagation of both inter- and intra-granular cracks destroy the rock microstructural integrity, and that cold water breaks the bond between grains and further widens thermal-induced cracks. When the temperature increases from 20 to 800 °C, the physical and mechanical properties of the two investigated rock types are both weakened, including the density, P-wave velocity, and compression strength. Rapid water cooling more significantly affects the rock mechanical properties than slow cooling owing to the intense thermal shock. The presence of thermal-induced cracks at high temperature significantly influences the rock homogeneity and elastic modulus, and the quenched specimens showed less brittle characteristics at high temperature. The DIC analysis indicates that thermal-induced cracks alter the local stress concentration around the simulated borehole and increase the unstable crack propagation regions owing to stress redistribution.
      PubDate: 2022-07-11
       
  • Propagation and geometry of multi-stage hydraulic fractures in anisotropic
           shales

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      Abstract: Abstract We analyze the efficiency of hydraulic fracturing operations by modeling reservoirs undergoing one and three hydraulic fracturing stages under various far-field stresses and injection rates using the eXtended finite element method. The reservoir is vertical transversely isotropic (VTI), and the effect of the degree of anisotropy on fracture propagation is analyzed. The numerical models are validated using the KGD model and experimental studies. We calculate the permeability of the fractured medium using the Gueguen and Dienes model, enhanced to account for the proppants' presence within the fractures. Our results show that the cracks expand towards the maximum principal stress in isotropic formations and kink towards the weakest plane in VTI formations as the mechanical contrast increases. This behavior tends to close the cracks and leads to a higher compressive load on the proppants. Moreover, cracks grow independently when the distance between cracks exceeds 12 m, and a planar propagation is observed for a distance above 20 m. The results also show that the fractured area and the resulting permeability are larger for close cluster spacing (< 12 m). This work allows the identification of the best fracturing scenario to optimally enhance the permeability of anisotropic unconventional reservoirs under different hydro-mechanical conditions.
      PubDate: 2022-07-06
       
 
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